Nov 2007 Update - this is my first attempt at a PSK31
interface. I have made several changes over the past years and
will write up something to update this page (but it is not done
yet). The K2 firmware has been updated to include an RTTY
mode, and an independent setting for the compression was added for this
mode, so the portion of this article dealing with asuring the
compression is at 1:1 is no longer applicable.

Original Article (circa 2001):

I have spent a short time so far on PSK31. At first, I just
connected a cable (with 1/8 inch plugs on both ends that I obtained
from Radio Shack) from my K2 Transceiver to my computer soundcard
line-in jack. This and the DigiPan and WinPSK programs enabled me to
copy PSK31 signals – I adjusted my sound card input volume controls and
the audio gain on my K2 so that I had a nice speckled blue background
in the waterfall display, and I was seeing FB print on my computer
screen.

It was not very long afterward that I wanted to transmit this PSK31
stuff too. Things got a bit more complex at that point. I had to put an
attenuator in the cable from the sound card output line to my
microphone. After some manipulation with values I got a respectable
signal into my K2 and I could transmit this digital stuff too, but I
was using manual PTT which made things a bit cumbersome going from
receive to transmit.

The next step was to automate the PTT as well. The instructions in
DigiPan and WinPSK showed a simple circuit – just a resistor, a diode
and a transistor. I hooked it up and hid all the parts into the shell
of my serial port connector with just one shielded wire going to my K2.
All I had to do is tell the PSK31 program to transmit and it all
happened!

After all that excitement one day, I wanted to use normal SSB – that
mode which uses the microphone, and here I had all these plugs in my K2
going to my computer that needed to be unplugged. Then and there I
decided that there should be an easier way, I needed a simple switch to
change from one mode to another to keep my frustration level on an even
keel.

I had a switch box on hand that was designed to switch serial port
lines from a computer to 2 devices (or the other way around). This
switch had a nice size box, a built in 9 pole 2 position switch and
connectors on the back. The inside had plenty of room for me to mount
everything including the microphone preamplifier.

My Grand Plan and Mods to the K2

In addition to putting all the switching into one box, I wanted to
end up with only one cable connection with my K2, and also to be able
to disable the speech compression of the K2 when I switched to PSK31
without thinking about changing it through the menu.

With version 2.x firmware and above, the need to use electrical
circuits to
disable the speech compression has been eliminated. If the RTTY
mode is
enabled, the K2 menu allows setting the speech compression for RTTY
mode
independently of SSB mode. If you chose not to enable RTTY mode,
or to
operate PSK31 in SSB mode instead, you may still want to use this
circuit to
disable the compression.

To accomplish that, I had to modify the K2 to add the headphone
audio through the microphone jack and I needed yet another signal
through the mic connector to disable the speech compression. Adding the
headphone audio was easy – that is mainly wires, but disabling the
speech compression was more challenging.

At first examination of the SSB schematic revealed that the COMP0
signal was active low which made it easy in principal – just short it
to ground to disable the compression. In practice things are not that
easy – I would be adding relatively long wires to ground it in my
switch box, and that would likely couple noise into this line at times
other than when it is grounded – some isolation was required.

I ended up with a simple circuit of 2 resistors and 2 transistors
configured as a non-inverting open collector switch that I could mount
on the SSB board. I was able to package them so they mounted right on
top of U3 with the tops of the transistors not much higher than the
adjacent electrolytic capacitors. This circuit was built with ‘flying
leads’ construction to minimize the package size. All the components
self support nicely and the lead lengths are very short. The result is
effective control of an analog signal line at the output and a digital
signal input that should have good noise immunity. If you have worked
with RTL digital components, you will likely recognize the circuit.

For my new audio output, I used a 82 ohm resistor and 2.2 mf
capacitor from the junction point of R35 and R36, duplicating half of
the components that normally go to the stereo headphone jack. I did not
want a fault in the external box to disable the normal K2 audio output.
These components were added below the RF board quite near the headphone
jack.

Microphone Header Configuration

My microphone header does not have any wires on it. I originally
connected everything straight across using jumpers normally used in
computers. My microphone wiring is identical to the wiring needed for
the future Elecraft microphone. To add the signals for headphone audio
and compression disable to my mic connector, I chose pins 4 and 5. Pin
5 was already unused, and I felt I could sacrifice both the UP and DOWN
button functions since my microphones didn’t use these anyway. That
left me with pin 3 reserved for any future needs. Your pin
configuration may be different if you are using another type microphone
and don’t want to alter your mic’s plug wiring.

To connect my new wires to the microphone header, I used more of the
computer jumpers. Some of the jumper types have an exposed part of the
metal connecting bar at the top with a small opening below that. I
soldered wire to the top and plugged it onto the header using only one
side. These connecting jumpers must only connect to the numbered side
of the header which then connect to the mic jack. The side of the
header with the signal names connects to circuits inside the K2 and
should be left open for these added circuits.

Schematics of the circuits added to the K2 are shown below with
pictures of my physical implementation.

This switchbox uses the components from a computer COM port switch.
Only 2 of the original connectors are used and only 4 of the 9 poles of
the switch. I mounted my preamplifier (details below) inside the box
with its two microphone inputs (only one is shown on the schematic). I
also added a jack for using an external footswitch for PTT (bonus – I
can now go QLF on CW).

The headphone jack is shown in the schematic as monaural, but should
actually be a stereo type with both the tip and ring wired in parallel.

The internal wiring can be seen above. The DB9 connector in the
lower left goes to the COM port and the PTT circuit is built using
‘flying leads’ construction. The RCA jacks above it are for the
soundcard connections. The one with the trimpot attached is for the
line out/K2 mic in signal. Adjust the trimpot for the correct audio
drive to the K2 with the computer’s volume control set to midpoint or
below.

The center DB9 connector is used for the cable to the K2 mic
connector. I used a shielded RS232 serial cable for that task. The
shield is grounded to the DB9 shell at the switchbox end and connected
to my pin 8 at the K2 end. Wire this connector to correspond to your
normal microphone wiring so you can use your microphone with either the
switchbox or directly on your K2.

The External PTT connector is the RCA jack mounted below the center
DB9.

The far end of the box contains the microphone jack (or in my case 2
microphone jacks), the headphone connector (1/4 inch type to match my
Heil MicroPro) and the microphone preamp mounted on the bottom of the
case. Additional photos follow.

My operating position is shown above. The two microphones are in the
foreground and my new PSK31 (as well as other soundcard generated
digital modes) switchbox appears to the left of the D-104 microphone
head. Those with sharp eyes can see another operating convenience item
to the right of and behind the switchbox. I used an old sound card to
extend the jacks near the operating position rather than pulling out
the rear side of the computer.

Noise Canceling with two Heil Microphones

The XYL gave me a Heil MicroPro Headset with the HC-4 cartridge for
Christmas (thank you Nancy). I already had a Heil HC-5 element in my
old D-104 mic frame. Both these mics needed a bit of amplification to
work reliably with my K2, so I needed to build a mic preamp too. The
December 2000 issue of QST came to the rescue with an article by WB9YBM
and N9BRL for a Noise Canceling Microphone on page 38. This article
presented a technique for using two electret mic elements to achieve a
noise canceling effect. I saw it as a design that would allow me to
have both microphones actively connected at the same time and possibly
do double duty as a noise canceling circuit. I built the amplifier,
connecting the input resistors (R1 and R10) to ground rather than the
supply voltage because my Heil mic elements didn’t need any voltage on
them like the original electrets do. My measurements showed that I did
achieve some noise canceling characteristics even though the amount was
highly dependent on the placement of the microphones. That was good
because the K2 does not implement an anti-VOX and I thought that the
noise canceling aspects would think the K2 speaker sound was also noise
and give me some anti-VOX action too. I am happy to report success, but
with the caveat that the amount is dependent on microphone placement
just as I expected. The overall gain of the amplifier was unity as
designed, and since I needed a bit of amplification, I changed the
emitter resistors for the two output transistors (R9 and R16) from 680
ohms to 330 ohms which gave me a gain of about 2.

I built the amplifier on a piece of circuit board with land patterns
cut out with a Dremel tool resulting in a pseudo Manhatten style
construction. I have ordered the punch from Harbor Freight to punch out
true Manhatten style pads, but it has not yet arrived. If such
construction practices are counter to your ambition, a circuit board is
available – please consult the original article.

One additional note – I had to add a 10 ohm series resistor and a 47
mf capacitor on the +5 volt supply lead to reduce some noise spikes
that caused me some trouble when I first connected the preamp.